The present invention relates to controls for electric power sources, and more particularly to a control system for substituting one source for another within an extremely short period of time.
With many types of electrical and electronic apparatus it is necessary that the apparatus remain in an energized state despite the failure of a regular power source. For this reason a number of systems have been devised for producing emergency or standby power in the event of the failure of the principal power source. Such systems as presently known generally fall into two categories.
In the first category, which may be termed "uninterruptible" supply system, a load is supplied directly through the "standby" source, which may for instance be an inverter powered by a battery. The principal or "line" power source constantly charges the battery through a primary (first) rectifier which supplies power to the inverter for energizing the load. When a failure of the principal source occurs power continues to flow from the battery into the inverter, so that no interruption of power to the load is experienced. Of course, after some period of time the battery becomes discharged inasmuch as it is no longer receiving charging current from the outside source. Although the so-called uninterruptible type of power supply ordinarily performs well, it is uneconomical for many installations inasmuch as the primary rectifier system must be large enough to supply all of the power needed by a load under all circumstances, and moreover must be constructed to provide extremely high reliability and durability inasmuch as the "standby" system must operate continuously and if not sufficiently durable can itself be a source of failure.
A second and less expensive approach to providing an alternative power supply is to provide a battery-operated "standby" system which does not operate until the principal power source fails. Typically, a small charging system is provided to keep the battery of the system fully charged. A sensor, relay or the like is coupled in circuit with the principal or "line" power source so that when the principal source fails the standby inverter is immediately energized. Unfortunately, while the latter approach is comparatively economical and straightforward, the switchover procedure which it necessitates causes interruptions in the continuity of power flow which are, for some types of loads, extremely significant.
With the increasing use of computers and other data processing machinery having electrically-energized memories or registers in which data is stored, assuring a continuity of supplied power is extremely important. Also of particular interest are lighting systems incorporating high intensity discharge (hereinafter (HID) lamps. Lamps of this type consideration commonly comprise an envelope filled with a vapor of mercury, sodium, or one or more metal halides which ionize in the presence of a current flow and produce illumination. Since their inception HID lamps have become increasingly popular, both for their lighting characteristics and for their high lumens per watt rating, which makes them relatively economical to operate.
While it is often desirable to use HID lamps for indoor lighting, and particularly in commercial buildings, modern lighting codes commonly require the presence of standby or emergency power sources to assure continued illumination in the face of power failures.
Unfortunately, while HID lamps are in most respects well suited for use in populated structures, once they have been extinguished they must either be allowed to cool for a period of approximately 15 or 20 minutes, or else an inordinately high voltage is required to relight them. The voltage required is such as to make it impractical to provide standby power sources capable of producing the necessary voltage. To make matters worse, HID lamps will commonly become extinguished within approximately 4 milliseconds after a power supply failure, depending upon the specific type of lamp and the nature of the vapor. The quenching time of the HID lamps is far more rapid than the operation of the switching means which are conventionally utilized in transferring power from an outside source to a standby unit. Accordingly, it will be appreciated that it would be highly desirable to provide a rapid transfer system which maintains the effective continuity of power flow to a load, obviating any detrimental effects on the load.
It is therefore an object of the present invention to provide a power transfer system which operates substantially faster than any prior art system.
It is another object to provide a transfer system which will effectively maintain the continuity of supplied power while providing isolation from a defective power source and enabling a localized standby power source.
Still another object is to provide an improved rapid power transfer system.
Another object is to provide a rapid transfer system which will maintain the illumination of HID lamps while decoupling the load from the principal power supply.